I was looking up some info about the NES Super Mario Bros. localization and came across this site that explains a Famicom-specific cart swapping technique for accessing the 256 'minus worlds' in SMB. Since the trick involves removing the cartridge while the game is playing, the CIC precludes duplicating it on an NES.

There are two techniques described in the article (scroll down to the 'Super Secret Worlds' section for more details):

- Remove the SMB cart during gameplay. Insert a Tennis cartridge, reset, play a bit, then remove the cart. Insert SMB, reset, then use the A+Start stage select to begin play in a corrupted world.

- Version 2 uses a similar method, but involves the Family BASIC cart, keyboard, and a snippet of BASIC code executed during play:

The author includes a subtitled Japanese video of the first technique, which looks legitimate. The technique is also backed up by significant documentation in several Japanese SMB guides in the 1980s. Apparently this trick was well known.

Anyone heard of this before? Technique 2 seems almost like a homemade Game Genie. Could this be replicated on a top-loader (or a CIC-disabled front-loader)?

If I'm not mistaken, this works simply because by hoding A+Start as you begin a game in SMB, it does not clear the values in ram associated with the current world. Normally, the value in ram will simply be the world you were previously in. By starting another game, which uses that address for a totally different reason, and pulling it and resetting, that value is never reset, so SMB uses whatever was there.

This should work with many games, with potentially different results - you may find yourself in a legitimate world like 2-8, even. On a toploader NES, this should work, as well as a CIC-disabled toaster.

Last edited by mikejmoffitt on Fri Jun 08, 2012 7:47 am, edited 1 time in total.

Turns out I had both Tennis and SMB. It took a bit of wrangling (my Tennis cart wasn't working well), but I can confirm this works on a top loader. Two of the minus worlds I arrived in crashed within a few seconds of play. The others were merely copies of valid levels.

I notice in the video from the article that, after removing the cart, the background tiles appear to slowly 'melt away.' I noticed this happening a few times for me too. What causes that effect?

And it was neat to swap carts mid-game and have their name tables update with tiles from the new pattern tables!

This probably wouldn't work on a toaster even with the CIC disabled just because it's difficult to insert and remove cartridges quickly.

The other important aspect of this trick is that Super Mario Bros only clears RAM on startup when any of the bytes used to store the score is greater than 10 or the value of the last byte of RAM is anything other thn $A5. Tennis, being another early Nintendo game, uses the same place to store the high score and the same signature byte, so the RAM is not cleared. The same trick might work with some of the other Nintendo black box games as well depending on how much code was reused.

I notice in the video from the article that, after removing the cart, the background tiles appear to slowly 'melt away.' I noticed this happening a few times for me too. What causes that effect?

And it was neat to swap carts mid-game and have their name tables update with tiles from the new pattern tables!

Yeah I see the same thing when swapping games on the NESDEV1. It effectively does the same thing by tristating the cart's data busses and CIRAM inputs. IDK if it's really what's going on, but I always attributed the 'melting' effect to bus capacitance holding values that leak away.

My famicom will do that with removed games. After a few minutes, where it starts to become unstable with the data, moving my hand closer and further from the cartridge slot very visibly affects the data. It's a neat theramin-type effect.

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